1. Field of Invention
The present invention relates to a fibrous sheet for forming a polymer gel electrolyte having ion conductivity, to a polymer gel electrolyte using such a sheet, and to a method for the manufacture thereof. More specifically, the present invention relates to a fibrous sheet for forming a polymer gel electrolyte having excellent ion conductivity and suitable as an electrolyte for various electronic devices such as lithium primary batteries, lithium secondary batteries, and electric double layer capacitors, to a polymer gel electrolyte using such a sheet, and to a method for the manufacture thereof.
2. Description of the Related Art
Lithium batteries containing a complex lithium oxide such as LiCoO.sub.2, LiMn.sub.2 O.sub.4, and LiNiO.sub.2 as a positive electrode active material, a Li alloy or a carbonaceous material capable of intercalating and deintercalating Li as a negative electrode active material, and a mixture of a nonaqueous solvent and an electrolyte salt as a battery electrolyte, demonstrate high performance and excellent rechargeable characteristic. For this reason they have found wide application as power sources for personal computers, cellular phones, minidisks, minicomponent systems, etc., and are being studied as environmentally friendly power sources for automobiles.
Nonaqueous electrolyte batteries that have been developed in the past used organic solvents such as an ethylene carbonate or a propylene carbonate for electrolytic solutions. For this reason, when such batteries were exposed to a high temperature atmosphere, they could be internally or externally short circuited which resulted in an increase of the battery temperature. There has been a drawbacks in that, in such a case, the electrolytic solution evaporates and the battery may explode or ignite.
Accordingly, polymer gel electrolytes that are easier to handle than the liquid electrolytes and which have a high ion conductivity and demonstrate no solvent evaporation have been studied as electrolytes which do not leak from lithium secondary batteries like the nonaqueous electrolytic solution. For example, Japanese Laid-open Patent Publication No. Hei7-245122 discloses the utilization of a gel prepared by crosslinking polyethylene oxide triol with a difunctional isocyanate, and Japanese Laid-open Patent Publication No. Hei6-96800 discloses the utilization of a gel-like crosslinked substance polyether polyol acrylate. Furthermore, J. Polymer Sci. Polymer Physics, Vol. 21, 1983, pp. 939-948 and Japanese Laid-open Patent Publication No. Hei8-264205 discloses a method for the preparation of a polymer gel electrolyte by mixing a small amount of powdered polyacrylonitrile with ethylene carbonate and propylene carbonate, then by dissolving an electrolyte in the mixture followed by mixing and casting the same.
Polymer gel electrolytes produced from crosslinked products such as polyesters and polyol acrylates, or from polyol and isocyanate adducts, have a low ion conductivity of about 10.sup.-5 S/cm and their performance is inadequate for battery applications. In order to increase the ion conductivity, it is necessary to decrease the crosslinking density of the gel electrolyte and to increase the electrolyte content. However, solid electrolytes manufactured by such a method have a decreased strength, which is undesirable. From the industrial standpoint, a low strength in gel electrolytes is a fatal disadvantage because it prohibits the application of a continuous battery assembly process. Furthermore, when using a method comprised of dissolving a powdered acrylonitrile polymer in a nonaqueous solvent and followed casting, the acrylonitrile polymer shows a poor solubility in nonaqueous solvents, and aggregates of the acrylonitrile polymer particles are easily formed in the dissolution process, which makes it impossible to produce a high-quality gel electrolyte sheet. (For example, if a polymer for forming gel electrolyte is placed into a heated solvent, only the surface of particles is swollen due to dissolution, and the particles having such a swollen layer stick to each other and form aggregates having a diameter from several millimeters to several tens of millimeters. Such particle associations are coated with a surface layer which is dissolved by the solvent and has a high resistance to solvent diffusion. This layer impedes the dissolution of the internal part of these associations. Furthermore, in such a state, these aggregates are extremely difficult to break and dissolve, even by applying a shear force to the solution by stirring.) In addition, a gel sheet manufactured using acrylonitrile polymer solutions has a non-uniform thickness and its electric properties can be easily degraded.
TECHNICAL PAPERS, ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, J. Electrochem. Soc., Vol. 142, No. 3, March 1995, p. 683, discloses a solid polymer electrolyte prepared by filling micropores in a polyethylene microporous membrane with a solid electrolyte manufactured from polytetraethylene glycol diacrylate. Although this solid polymer electrolyte shows a high strength, its ion conductivity is as low as 2.times.10.sup.-4 S/cm and its characteristics are inadequate because of its physical structure and properties of the polymer electrolyte used for its manufacture. Furthermore, film-like solid polymer electrolytes prepared by impregnating filter papers or nonwoven fabrics made of synthetic fibers with the above-described solid polymer electrolyte are also known, but thin films of such solid electrolytes are difficult to be produced, and a uniform film thickness cannot be obtained. Moreover, its ion conductivity is not higher than 10.sup.-4 S/cm and its characteristics are also inadequate.